Ventilation is the least visible part of a roof and the one most likely to void the warranty on everything above it. It is also the part most often got wrong on otherwise competent installations.
The rule almost nobody is told
Read a shingle manufacturer’s warranty and you will find ventilation treated as a condition, not a footnote. CertainTeed’s asphalt shingle warranty is the bluntest: shingles applied to an inadequately ventilated or non-ventilated deck are subject to a reduced limited warranty period of ten years, and do not qualify for its SureStart protection. In their published table, that same roof loses wind coverage and algae coverage as well.
So a lifetime shingle on a poorly ventilated attic is a ten-year shingle. Nothing about the product changed — only the air underneath it.
GAF words it differently and lands in the same place: “Failure to install adequate ventilation is NOT an application error of your GAF Products and is not covered under this Limited Warranty.” Owens Corning makes a ventilated deck an outright eligibility condition for its Platinum warranty.
Balance means intake, not exhaust
The instinct when an attic runs hot is to add exhaust. That is usually the wrong move, and sometimes actively harmful.
Air has to come in before it can go out. The Asphalt Roofing Manufacturers Association states the rule plainly: intake vents should provide 50 to 60 percent of the open venting area and exhaust 40 to 50 percent — the intake amount should always exceed the exhaust amount. GAF says the same from the other direction: ridge and hip exhaust should not exceed soffit intake.
When exhaust greatly outweighs intake, the NRCA notes something worse than poor airflow: the attic can go to slight negative pressure, and start pulling conditioned air up out of your house through every gap in the ceiling — recessed lights, bath fan housings, plumbing penetrations.
The most common cause of intake starvation is not a missing vent. It is blown insulation pushed out over the eaves, sealing the soffit vents from the inside. The residential code requires at least a one-inch gap between insulation and sheathing at the vent for this exact reason. Baffles are cheap. Finding out you never had them is not.
Do not mix exhaust types
This is the one that surprises people, because both vents are working correctly on their own.
GAF’s technical bulletin on attic ventilation is unambiguous: “DO NOT use multiple vent systems.” If continuous soffit and ridge vents are used, power fans, roof louvers and other static exhaust vents should be removed or disconnected, and gable vents closed. Combining them, GAF warns, “could result in reverse airflow that could result in water leakage into the attic.”
The mechanism is simple. Air takes the path of least resistance, and with two exhaust openings on one attic, that path becomes the short hop between them rather than the long run from eave to ridge. The lower attic stops venting altogether. And one of the two exhaust vents becomes an unintended intake — which means it is now drawing weather in through an opening designed only to let air out.
The NRCA reaches the same conclusion: static vents alongside a ridge vent “should be avoided — the combination may render the ridge vent ineffective because the static vents create a ‘short circuit’ of the intended air flow.”
Powered attic fans
Worth a separate warning. The US Department of Energy’s Building America programme notes that attic fans create a negative pressure zone that draws conditioned air out of the living space, and that combustion appliances other than sealed-combustion units “may not draft as well or could potentially backdraft.” Measured research from ACEEE put the average draw at 231 cfm of conditioned air pulled out of the house. They frequently use more electricity than they save.
What to ask for
When you are quoted a roof, ask two questions:
- How much net free intake area does this attic actually have — not how much exhaust is going on
- What is being removed — if a ridge vent is going on, the box vents and any powered fan should be coming off
If the answer to the first is a shrug, the ventilation on your new roof is being guessed at, and your warranty is riding on the guess.
Sources
- CertainTeed asphalt shingle limited warranty — the ten-year reduction
- GAF Technical Advisory Bulletin R-120 — proper attic ventilation, mixed systems
- ARMA attic ventilation best practices — the 50–60% intake rule
- DOE Building America: attic ventilation fans — powered ventilator depressurisation